1. Field of the Invention
The present invention relates to a wire-protecting apparatus for use with automobiles that protects electrical components and electrical wirings used for cars.
2. Description of the Related Art
Conventionally, electrical components mounted at various locations of an automotive vehicle are connected to each other by the use of a junction box where a number of electrical and electronic components such as relays and fuses are collectively housed.
FIG. 12 shows a schematic diagram of a system disclosed in Japanese Laid-open Patent No. 61-188,246, which system protects electrical components and wires from overcurrent caused by troubles such as a short circuit. A lighting circuit a for lighting lamps L is provided with a fuse b and a potential difference detector c. The potential difference developed across a current detecting resistor d in the detector c is directed over a indicator circuit line e to an indicator f. Indicator lamp g is lighted up by the potential difference clearly indicating an abnormality such as a burning-out of a lamp. However, the system has a disadvantage in that once a fuse b blows out due to overcurrent, the system failure cannot be remedied the fuse b is replaced with a good one. Since the occurrence of overcurrent is displayed only after the fuse has blown out, problems cannot be detected before the fuse blows out.
FIG. 6 shows a prior art motor-driving circuit. The motor is in series with a fuse 13 which protects wires 12 when the motor circuit is short-circuited. The motor 14 is provided with a motor protecting element 15 which protects the motor 14 from burning out when the motor is locked.
FIG. 7 shows characteristic curves showing the relation between time and the current flowing through the circuit in FIG. 6. In FIG. 7, curve W represents the smoke characteristic of the wire 12, curve F the melting characteristic of the fuse 13, curve J the current waveform when the motor is looked. Curve T represents a time duration before the lock current is controlled by the motor protecting element 15. The wire protecting apparatus is designed so that the fuse 13 blows out before the wires begin to smoke. Thus, curve W does not cross curve F. The fuse is selected so that curve J crosses curve F. In this manner, the fuse 13 and motor protecting element 15 have different tasks. This indicates that the circuit needs two wire-protecting means which impose deficiencies such as large space required, large size of components, and more weight.
FIG. 11 shows a conventional door control relay circuit having a wire-protecting circuit based on a fuse 17 and a circuit breaker 18. There are also shown a door lock motor 19 and a relay unit 20.
During start-up of the motor starts as well as when the motor is locked, a considerably large current compared with a rated current flows through the wire. In order to carry this large current, conductors and associated components in the system package must be of large capacity. This inevitably increases the size and weight of both the leads and associated components of the circuit, and the excessive heat due to a large current may possibly cause the printed circuit board to burn out.